Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having a high-energy density and a high-discharge voltage. These lithium secondary batteries are also commercially available and widely used.
Further, increased environmental concern has drawn a great deal of intensive research on electric vehicles (EVs) and hybrid electric vehicles (HEVs) which are capable of replacing fossil fuel-driven vehicles such as gasoline vehicles and diesel vehicles, one of the primary causes of air pollution. Although nickel-hydrogen (Ni—H2) secondary batteries are largely employed as power sources for EVs and HEVs, numerous studies have been actively made to use lithium secondary batteries having a high-energy density and a high-discharge voltage, consequently with some commercialization outputs.
Generally, the lithium secondary battery is comprised of a structure having an electrode assembly composed of a cathode, an anode and a porous separator disposed between the cathode and the anode, and with impregnation of the electrode assembly with a non-aqueous electrolyte containing a lithium salt, wherein the cathode and the anode are fabricated by applying electrode active materials to current collectors. As the cathode active material, lithium cobalt oxides, lithium manganese oxides, lithium nickel oxides, lithium composite oxides and the like are primarily used. As the anode active material, carbon-based materials are usually used.
In order to use such a lithium secondary battery as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs), the battery is required to have the performance that is capable of operating even under more severe conditions than those under which mobile phones, notebook computers, personal digital assistants (PDAs) and the like operate. For example, since vehicles must be operable even under low-temperature conditions such as winter seasons, representative examples of requirements necessary for the aforementioned power source may include excellent power output characteristics at low temperatures. If the power source cannot provide sufficient power output at low temperatures, driving of a power system is not going smoothly. Even further, if the power output does not reach a minimum power output necessary for starting of the vehicle engine, the operation of the vehicle itself may be impossible.
Attempts to improve low-temperature power output characteristics of the lithium secondary battery have been made primarily toward performance improvements in electrolytes and anode materials. As conventional arts relating to improvements in the performance of anode materials, there have been proposed a technology of enhancing power output characteristics by doping a surface of an anode active material with a conductive metal to thereby facilitate migration of ions and electrons, and a technology of using active materials having excellent rate properties instead of carbon-based materials, as the anode active material. However, the technology of doping the surface of the anode active material with the metal suffers from fundamental problems associated with a difficulty to achieve a desired level of doping in the mass production process. In addition, the technology using anode active materials having excellent rate properties just provides improvements in low-temperature properties at the sacrifice of high-temperature properties and battery capacity, that is, suffers from disadvantages of decreased high-temperature performance and capacity.
Therefore, there is a strong need in the art for the development of a technique which is capable of easily and conveniently improving low-temperature power output characteristics while not causing substantial decreases in the high-temperature properties and battery capacity.